The long-term goal of Dr. Laura Anne Lowery is to obtain a tenure-track faculty position at a research university and develop a comprehensive, multi-faceted research program that investigates the logic by which guidance information is integrated at the level of cytoskeletal dynamics during axon pathfinding. To this end, she has constructed an extensive career development and research training plan which will facilitate her success and complement her previous training experiences. She received her BS and MS in biology from UCSD, where she worked with Dr. William Schafer on the neural circuitry controlling C. elegans behavior. This work resulted in two papers (including first-author in Journal of Neurobiology). She received her PhD in Biology at MIT under the mentorship of Dr. Hazel Sive. Supported by a pre-doctoral NRSA, she made significant progress defining the genes essential for early brain morphogenesis, including the identification of several genes required for normal neurogenesis and axon pathway formation. This work resulted in five first- author publications in journals such as Development. In July 2008, Dr. Lowery joined the Van Vactor lab in the Department of Cell Biology at Harvard Medical School, where she began a project to identify new interactors of an intriguing cytoskeletal regulator that functions downstream of axon guidance cues, called CLASP. This work, supported by a post-doctoral NRSA, has thus far resulted in 2 first-author publications (in Genetics and Nature Reviews). Dr. Lowery's immediate goal is to gain new expertise in quantitative cytoskeletal imaging and analysis using Xenopus growth cones, in order to investigate the roles of specific microtubule regulators during axon guidance. While in the mentored K99 phase, Dr. Lowery will continue to benefit from the mentorship of Dr. Van Vactor, a leader in the field of genetic analysis of axonal growth and guidance. Additionally, Dr. Lowery will receive new training and support from co-mentor Dr. Gaudenz Danuser, one of the world's leaders in quantitative cytoskeletal analysis. Both Drs. Van Vactor and Danuser have excellent mentoring records and are committed to fostering Dr. Lowery's training and independence. This environment is an ideal setting for her transition to independence, as Harvard Medical School is one of the strongest biomedical research facilities in the country and is perfectly suited to facilitate the goals in this proposal. Her development will be enhanced by additional microscopy and computation courses, as well as support from an advisory committee of expert investigators of axon guidance and the cytoskeleton. The new skills, techniques, and experimental data she acquires during the K99 phase (Aims 1, 2) are essential to the research planned for the independent R00 phase (Aim 3). The research objective in this application is to determine how a specific group of microtubule 'plus-end tracking proteins' (+TIPs) localize, interact, and function, within the growth cone downstream of guidance cue signaling. Initial work has identified +TIP XMAP215 and its co-factor Maskin as potent antagonists of the +TIP and Abl signaling substrate, CLASP. Furthermore, XMAP215 and Maskin are required for accurate axon guidance decisions in vivo, and XMAP215 antagonizes Abl's in vivo axon guidance function. These preliminary findings, combined with knowledge from non-neuronal studies of +TIP function, have led to the working model that, within the growth cone, XMAP215 and Maskin interact with microtubules (MTs) in a functionally-distinct manner compared to CLASP, and that Abl signaling leads to differences in the ability of these +TIPs to interact with each other and with microtubules, thereby driving changes in cytoskeletal dynamics and growth cone directionality downstream of guidance cues. This will be tested using a combination of quantitative imaging, genetic manipulations, and biochemical approaches, to pursue three specific aims. Aim 1) How do +TIPs behave and co-localize with each other and with microtubules inside the growth cone? +TIP localization and MT dynamic instability parameters will be quantified using computational analysis, following acquisition of high-resolution live imaging data of +TIPs and MTs within cultured Xenopus growth cones. Aim 2) How does +TIP function influence MT dynamics and growth cone motility? This aim will use loss-of-function and gain-of-function genetic strategies in Xenopus combined with the imaging platform established in Aim 1 to identify the functional roles of XMAP215 and Maskin, compared to CLASP, within the growth cone. Aim 3) How is +TIP function within the growth cone regulated by upstream guidance signaling? In part 3A, biochemical experiments using Xenopus embryonic lysates will be performed to assess the regulation of +TIP binding events in vitro and to determine the structural domains that modulate those interactions. In part 3B, high-resolution live imaging will allow visualization of +TIP/MT interactions as the growth cone encounters guidance cues in culture, as well as after direction manipulation of Abl signaling. This approach is innovative because it will, for the first time, combine state-of-the-art imaging and analysis tools to pioneer the elucidation of quantitative global MT and +TIP behavior within cultured growth cones during decision-making events. The proposed research is significant because it is an important step in a continuum of research that will illuminate how the growth cone cytoskeleton is coordinated during axon guidance, the knowledge of which may eventually be applied to understanding the basis of neurodevelopmental and mental health disorders.